On systems which support the multisample extension
(SGIS_multisample), the per-fragment sample mask may be used to
change the transparency of an object. This method is basically
identical to screen-door transparency described in Section
12.1, but at a sub-pixel (fragment) level.

One technique involves GL_ SAMPLE_ALPHA_TO_MASK_SGIS. If
transparent objects in a scene do not overlap,
GL_ SAMPLE_ALPHA_TO_MASK_SGIS may be used. This parameter
causes the alpha of a fragment to be mapped to a sample mask which
will be bitwise ANDed with the fragment's mask. The value of the
generated sample mask is implementation-dependent and is a function of
the pixel location and the fragment's alpha value. If two objects
were drawn at the same location with the same transparency, the sample
mask would be the same and the same samples would be touched. If two
objects were drawn at the same location with different transparencies,
results may or may not be acceptable.

The simplest technique is to use the glSampleMaskSGIS() command
to set the value of the GL_ SAMPLE_MASK_SGIS. This value is used
to generate a temporary mask which is bitwise ANDed with the
fragment's mask. Again, results may not be correct if transparent
objects overlap.

Currently, SGIS_multisample is supported by Silicon Graphics and
Hewlett Packard.